The games plants play: the defensive strategies of Acacia nigrescens and Acacia tortilis with respect to the interaction between elephants and fire

African savannas are shaped by the interaction of two ecological factors: African elephants (Loxodonta africana) and fires. The response of individual Acacias to megaherbivores and fire regimes is dependent on their life history strategies. The resulting architecture of trees is a trade-off between a strategy of growing tall to escape low-lying fires or of developing thick, insulating bark to protect the internal structure from heat damage. Allometric studies suggest that there is a strong relationship between trunk diameter and average bark thickness, as well as trunk diameter and height, in Acacia nigrescens and Acacia tortilis. When both have similar bark thicknesses, A. nigrescens tends to grow taller and burn at higher temperatures than A. tortilis. The combination of bark stripping and field burns show that elephants can have dramatic effects on the survival abilities of trees during fires and that Acacias have varied life history strategies in response. Severely debarked (over 50%) A. nigrescens have greater odds of experiencing interruptions in the water column, as reflected in increased leaf discoloration and wilting, whereas A. tortilis shows no effect within the same time period. This suggests that A. nigrescens copes with fire and megafaunal herbivory by increasing canopy height and A. tortilis copes by investing in bark with greater insular ability to protect against fire and in mechanical defences to protect against herbivory